1. Field of the Invention
The present invention relates to a method of stress inducing transformation of austenite stainless steel and methods of producing magnetic members and composite magnetic members.
2. Description of the Related Art
At present, austenite stainless steel is widely used in various fields of railway vehicles to kitchen utensils for domestic use. Therefore, great importance is attached to the mechanical property of austenite stainless steel. Concerning austenite stainless steel, the following are well known. When austenite stainless steel is subjected to cold working in a temperature range from the point Ms to the point Md, the martensite phase is generated from the austenite phase which is a mother phase, so that the stress induced-martensite transformation is caused. In this case, the point Ms is an upper limit temperature at which martensite is generated by the isothermal transformation, and the point Md is an upper limit temperature at which martensite is generated by the stress inducing transformation. In this case, the above austenite phase is an fcc phase (face centered cubic phase). On the other hand, almost all of the above stress induced-martensite phase is composed of an xcex1xe2x80x2 martensite phase of the bcc phase (body-centered cubic phase), and a very small amount of the xcex5xe2x80x2 martensite phase of the hcp phase (hexagonal close-packed phase) is contained. The stress induced-martensite phase is defined as the aforementioned xcex1xe2x80x2 martensite phase in this specification, hereinafter.
In the case of a stress inducing martensite transformation, in accordance with increase in an amount of stress induced-martensite, there is a possibility that hardness and brittleness are increased and the mechanical property is changed.
However, as described above, the crystal structure of the austenite phase is different from that of the stress induced-martensite phase. Therefore, the austenite phase stainless steel is a non-magnetic member, and the stress induced-martensite phase stainless steel is a ferromagnetic member, that is, their magnetic properties are greatly different from each other.
Accordingly, when austenite stainless steel is used for a magnetic member or a composite magnetic member described later, it is very effective to increase a ratio of stress induced-ferromagnetic martensite phase.
On the other hand, according to the conventional producing method disclosed in Japanese Unexamined Patent Publication Nos. 7-11397 and 8-3643, it is impossible to increase the magnetic flux density B4000 to a high magnetic level not less than 0.8T (tesla), wherein the magnetic flux density B4000 is defined as a magnetic flux density in the case of applying a magnetic field with an intensity of 4000 A/m.
The reason why it is impossible to increase the magnetic flux density B4000 to a high magnetic level not less than 0.8 T (tesla) is considered as follows. An amount of strain which can be given to the magnetic member or the composite magnetic member is restricted by the limit at break and the shape of the member. According to the conventional cold working method, even if the maximum strain is given to the magnetic member or the composite magnetic member, a ratio of the generated stress induced-martensite is still low.
For the above reasons, there is a demand for developing a method of positively generating a large amount of stress induced-martensite, that is, there is a demand for developing a method of increasing an amount of the generation of stress induced-martensite with respect to an amount of the strain given to the magnetic member or the composite magnetic member.
Concerning the basic investigation with respect to the method of stress inducing transformation, for example, xe2x80x9cTransformation Induced by Working of SUS304 in Various Stress Conditionsxe2x80x9d was reported in the Spring Lecture Meeting of Plastic Working held in 1995. However, even according to the above investigations, it was impossible to develop the method of generating stress induced-martensite at a high ratio.
In order to solve the above problems, it is a first object of the present invention to provide a method of stress inducing transformation by which stress induced-martensite can be generated in austenite stainless steel at a high ratio of generation, and to provide a method of producing a magnetic member or composite magnetic member, the ferromagnetic property of which is high.
Further, for example, in a device such as an electromagnetic valve having a magnetic circuit, it is necessary to provide parts in which ferromagnetic and non-magnetic portions are integrated with each other. In order to produce such parts having both ferromagnetic and non-magnetic portions, for example, ferromagnetic and non-magnetic parts are separately produced, and then they are integrally connected with each other. However, according to the above production method, the durability of the connecting portion of the ferromagnetic part with the non-magnetic part is not so high, and further the production cost increases.
On the other hand, Japanese Unexamined Patent Publication No. 8-3643 discloses a composite magnetic member and a production method thereof in which ferromagnetic and non-magnetic portions are contiguously formed without having a connecting portion.
As shown in an embodiment described later, the above composite magnetic member can be provided as follows. Austenite alloy steel of a specific composition is used. This austenite alloy steel is subjected to cold working in a predetermined condition so as to generate stress induced-martensite. In this way, the austenite alloy steel is made to be ferromagnetic. After that, desired portions are subjected to solution heat treatment, so that these portions can be made to be non-magnetic.
For example, as shown in FIGS. 22A to 22D, there is provided a composite magnetic member 6 in which the main body is composed of a ferromagnetic portion 2 and the opening side portion is composed of a non-magnetic portion 3. In order to produce the above composite magnetic member 6, first, as shown in FIGS. 15A to 15F explained later, a plate 101 of austenite alloy steel is subjected to pressing by a plurality of times. In this way, the austenite alloy steel plate 101 is formed into a U-shaped member 106 by cold working. Due to the above cold working, stress induced-martensite is generated in the entire U-shaped member 106. Therefore, the entire U-shaped member 106 becomes ferromagnetic. Next, as shown in FIGS. 22A and 22B, the opening side portion of the U-shaped member 106 is subjected to solution annealing by a high frequency induction heating unit 98. Due to the above high frequency induction heating, the opening side portion of the U-shaped member 106 is made to be austenite, that is, a non-magnetic portion 3.
The thus obtained composite magnetic member 6 is excellent in the magnetic property. For example, the magnetic flux density B4000 (the magnetic flux density at H=4000 A/m) of the ferromagnetic portion is not less than 0.3T, and the specific permeability of the non-magnetic portion g is lower than 1.2.
However, the following problems may be encountered in the above conventional composite magnetic member 6.
As shown in FIG. 23, stress corrosion cracks 99 tend to occur in the non-magnetic portion 3 close to the boundary between the non-magnetic portion 3 and the ferromagnetic portion 2.
The reason why stress corrosion cracks 99 tend to occur is considered as follows.
As described above, the conventional composite magnetic member 6 is composed of the ferromagnetic portion 2 made of martensite and the non-magnetic portion 3 made of austenite. The crystal structure of austenite and that of martensite are different from each other. Therefore, the density of austenite and that of martensite are different from each other. For the above reasons, the volume of martensite is larger than that of austenite by 3% when the weight of martensite is the same as that of austenite.
In the conventional composite magnetic member 6, material of austenite is used. This material of austenite is transformed into martensite so as to form the ferromagnetic portion 2. Then, a portion of the ferromagnetic portion 2 made of martensite is returned to austenite, so that the non-magnetic portion 3 can be formed. Therefore, as shown in FIGS. 22C and 22D, only the non-magnetic portion 3 is reduced in its volume by 3% compared with the volume of the ferromagnetic portion 2. As a result, residual tensile stress is generated in a portion of the non-magnetic portion 3 close to the boundary between the non-magnetic portion 3 and the ferromagnetic portion 2. It is considered that the generation of this residual tensile stress greatly deteriorates the stress corrosion cracking resistance property.
On the other hand, there is provided another method. As shown in FIGS. 24A to 24C, after the completion of high frequency induction heating for making a portion of the composite magnetic member 6 to be non-magnetic, a punch 96 is forced in the inside of the composite magnetic member 6 so as to expand the non-magnetic portion 3. In this way, the non-magnetic portion 3 is plastically deformed, so that the above residual tensile stress can be removed. However, according to the above method, the following problems may be encountered. As shown in FIGS. 25A to 25C, the size of expanding the non-magnetic portion 3 becomes too large (shown in FIG. 25A) or too small (shown in FIG. 25C), that is, it is difficult to completely control the intensity of residual stress. In order to form the non-magnetic portion 3 into the most appropriate shape as shown in FIG. 25B, it is necessary to control the outer diameter of the punch 96 at a high level of accuracy of 0.01 mm, which is very difficult.
Another conventional method of removing the residual stress is a method of annealing a portion at which the residual tensile stress has been generated. However, in order to completely remove the residual tensile stress generated in the portion close to the boundary between the non-magnetic portion 3 and the ferromagnetic portion 2, it is necessary to anneal the entire composite magnetic member. When the entire composite magnetic member is annealed, the ferromagnetic portion is changed into a non-magnetic portion. Since the performance of the ferromagnetic portion must be maintained in the composite magnetic member, it is impossible to apply the above method.
In view of the above conventional problems, the second object of the present invention is to provide a composite magnetic member and a production method thereof by which the performance of the ferromagnetic portion and the non-magnetic portion can be maintained and it is possible to ensure a high stress corrosion cracking resistance property, as well as to provide an electromagnetic valve made of the above composite magnetic member.
(First Aspect of the Invention)
According to claim 1, the present invention is to provide a method of stress induced-transformation of austenite stainless steel, comprising the step of conducting cold working on a material of austenite stainless steel in a temperature range not lower than the point Ms and not higher than the point Md so as to transform the austenite phase into the stress induced-martensite phase, wherein the cold working is a biaxial tensing.
The most remarkable point in the above embodiment is to conduct a biaxial tensing as the cold working. In this case, the biaxial tensing is defined as a work such as a bulging in which tensile stress is give to material in the biaxial directions which are different from each other, and the material is elongated in the direction of the tensile stress and is shrinked in the direction perpendicular to the direction of tensile stress.
Examples of the above biaxial tensing are: bulging described above (including various methods in which metallic dies, hydraulic pressure, rubber dies and rollers are used), expanding, electromagnetic forming (explosive forming), and incremental forming.
In this case, the number of conducting the biaxial tensing may be one or plural according to the object. Alternatively, different working methods may be combined, and working may be conducted by a plurality of times.
The above biaxial tensing is conducted in a temperature range not lower than the point Ms and not higher than the point Md. When the temperature is lower than the point Ms, there is caused a problem in which martensite is generated by isothermal transformation caused only by lowering the temperature without conducting any working. Therefore, it is impossible to generate stress induced-martensite at a high ratio. On the other hand, when the temperature is higher than Md, there is caused a problem in which a strain is simply given to the austenite phase and no stress induced-martensite is generated.
Next, the mode of operation of this embodiment will be explained as follows.
According to the method of stress inducing transformation of austenite stainless steel of this embodiment, a biaxial tensing is conducted as the cold working. Therefore, it is possible to remarkably enhance a ratio of the generation of stress induced-martensite compared with a uniaxial or biaxial compression working or a uniaxial tensing (shown in Example 1).
The reason why a ratio of the generation of martensite induced by working can be remarkably enhanced is considered as follows.
Since the phase of stress induced-martensite contains the bcc phase as described above, a volume per unit weight of stress induced-martensite is larger than that of the phase of austenite of the fcc phase. For this reason, the stress induced-martensite transformation is accompanied by an increase of volume.
On the other hand, various types of cold working cause the stress induced-transformation. The aforementioned biaxial tensing is a method of working by which the volume of material can be increased at the largest rate.
Therefore, in this embodiment, the biaxial tensing functions not only as a cold working to cause the stress induced-transformation but also as a working to facilitate an increase of volume caused when the austenite phase is transformed into the stress induced-martensite phase. Accordingly, in the present invention, it is possible to remarkably increase a ratio of the generation of stress induced-martensite compared with other types of cold working such as compression working.
Therefore, according to the present invention, it is possible to provide a method of stress inducing transformation by which stress induced-martensite can be generated at a high generation ratio in austenite stainless steel.
(Second Aspect of the Invention)
There is provided an explanation of the method of producing a magnetic member having a high ferromagnetic property, wherein the above method of stress inducing transformation of austenite stainless steel is used.
According to the embodiment described in claim 2, the present invention is to provide a method of producing a magnetic member, comprising the step of conducting cold working on a material of austenite stainless steel in a temperature range not lower than the point Ms and not higher than the point Md so as to stress inducing transform the non-magnetic austenite phase into the stress induced-ferromagnetic martensite phase, wherein the cold working is a biaxial tensing.
According to this aspect, it is possible to produce a magnetic member having a high ferromagnetic property by utilizing a physical property that the stress induced-martensite phase is a ferromagnetic body. From the physical viewpoint, the transformation from the austenite phase to the stress induced-martensite phase is the same as the transformation from the non-magnetic body to the ferromagnetic body. For the above reasons, this aspect according to claim 2 is substantially the same as the embodiment according to claim 1.
Therefore, according to this aspect, when the biaxial tensing is conducted as the above cold working, by the same effect of the aspect according to claim 1, it is possible to generate stress induced-martensite at a high ratio of generation. Consequently, it is possible to easily obtain a magnetic member having a high magnetic property.
For the above reasons, when a composition of material and an amount of strain caused by the biaxial tensing are appropriately determined, it is possible to obtain a magnetic member having a very high ferromagnetic property, the magnetic flux density B4000 of which reaches a value not lower than 0.8T (shown in Example 3).
(Third Aspect of the Invention)
There is provided an explanation of the method of producing a composite magnetic member, wherein the above aspect according to claim 2 is used.
According to the aspect described in claim 3, the present invention is to provide a method of producing a composite magnetic member, comprising the steps of: conducting cold working on a material of austenite stainless steel in a temperature range not lower than the point Ms and not higher than the point Md so as to transform the non-magnetic austenite phase into the stress induced-ferromagnetic martensite phase and form a ferromagnetic portion; and conducting a stress inducing treatment on a portion of said ferromagnetic portion so as to form a non-magnetic portion of the austenite phase, to thereby form a composite magnetic member comprising the ferromagnetic portion and the non-magnetic portion contiguous to each other, wherein the cold working is a biaxial tensing.
The most remarkable point of this invention is described below. When the biaxial tensing is conducted as described above, stress induced-martensite is generated so as to form a ferromagnetic portion. Then, a portion of the thus formed ferromagnetic portion is subjected to a solution heat treatment so as to form a non-magnetic portion.
By the above solution heat treatment, only a portion of the ferromagnetic portion to be changed into a non-magnetic portion is heated to a temperature not lower than the transformation temperature of austenite. Examples of the means for conducting the solution heat treatment are high frequency induction annealing and laser beam machining.
It is preferable that the solution heat treatment is conducted in a short period of time not longer than 10 seconds. Due to the foregoing, it is possible to maintain the crystal grain size of austenite to be not more than 30 xcexcm, so that the specific magnetic permeability can be sufficiently reduced. On the other hand, when the solution heat treatment is conducted over a period of time exceeding 10 seconds, there is caused a problem in which the austenite structure becomes coarse.
In this case, the composite magnetic member is defined as a member in which the ferromagnetic portion and the non-magnetic portion are contiguous to each other in one body. In the above composite magnetic member, it is unnecessary to provide a connecting portion to connect the ferromagnetic portion with the non-magnetic portion. Accordingly, the thus composed composite magnetic member can be utilized as a very excellent member in the durability and the production cost to compose a magnetic circuit. For the above reasons, as described in the prior art, various producing methods of producing composite magnetic members are disclosed. The present invention aims to provide a method of producing a composite magnetic member having a ferromagnetic portion, the ferromagnetic property of which is higher than that of a composite magnetic member produced by the method of the prior art.
Next, the mode of operation of this embodiment will be explained below.
In the method of producing the composite magnetic member of this embodiment, the biaxial tensing is used as a means for forming the above ferromagnetic portion. As described above, a ratio of the generation of stress induced-martensite of this embodiment is remarkably higher than that of other methods. Therefore, it is possible to obtain a ferromagnetic portion, the ferromagnetic property of which is very high.
In the same manner as that of the embodiment according to claim 2, when a composition of material and an amount of strain caused by the biaxial tensing are appropriately determined, it is possible for this ferromagnetic portion to have a very high ferromagnetic property, the magnetic flux density B4000 of which reaches a value not lower than 0.8T (shown in Example 3).
In this embodiment, as described above, a portion of the ferromagnetic portion is subjected to a solution heat treatment. Due to the foregoing solution heat treatment, the heat treated portion is easily returned to the austenite phase, that is, the heat treated ferromagnetic portion is changed into a non-magnetic portion.
For the above reasons, according to this embodiment, it is possible to produce a composite magnetic member in which a ferromagnetic portion, the ferromagnetic property of which is very high, and a non-magnetic portion are continuously formed in one member.
As shown in the embodiment according to claim 4, concerning the cold working, it is preferable that a uniaxial compression working or a biaxial compression working is conducted after the above biaxial tensing. In the above case, it is possible to increase a total amount of strain given to the above material, and further it is possible to provide a ferromagnetic portion, the ferromagnetic property of which is high. In general, when a total amount of strain is large in a cold working, an amount of the generation of stress induced-martensite is increased. Therefore, it is very effective that a compression working, by which a relatively large amount of strain can be provided, is further given to the material after the completion of a biaxial tensing by which only a relatively small amount of stain can be provided.
Examples of the above uniaxial compression working or the biaxial compression working are: spinning, swaging, drawing with a metallic die, rolling, cold forging, ironing, drawing, extruding, and bending with a metallic die.
In this case, the number of conducting the uniaxial compression working or the biaxial compression working may be one or plural according to the object. Alternatively, different working methods may be combined, and working may be conducted by a plurality of times.
As described in the embodiment according to claim 5, it is preferable that the above cold working is conducted while it is divided into a plurality of stages. Due to the foregoing, it is possible to suppress a rise of temperature of the material when cold working is conducted. Therefore, it is possible to conduct a cold working in a temperature range not lower than the point Ms and not higher than the point Md.
As described in the embodiment according to claim 6, the above cold working may be conducted while the material is forcibly cooled. Also, in this case, it is possible to conduct a cold working in a temperature range not lower than the point Ms and not higher than the point Md.
As described in the embodiment according to claim 7, it is preferable that the above material is an austenite stainless steel, the composition of which is defined as follows. C is not more than 0.6 weight %, Cr is 12 to 19 weight %, Ni is 6 to 12 weight %, Mn is not more than 2 weight %, Mo is not more than 2 weight %, Nb is not more than 1 weight %, and the residual portion is composed of Fe and inevitable impurities, wherein Hirayama""s Equivalent Heq=[Ni%]+1.05 [Mn%]+0.65 [Cr%]+0.35 [Si%]+12.6 [C%] is 20 to 23%, and the nickel equivalent Nieq=[Ni%]+30 [C%]+0.5 [Mn%] is 9 to 12%, and the chromium equivalent Creq=[Cr%]+[Mo%]+1.5 [Si%]+0.5 [Nb%] is 16 to 19%.
The reason why C is not more than 0.6% in the above composition of the material is described as follows. When the carbon content exceeds 0.6%, an amount of carbide is increased, and the working property is lowered. The reason why an amount of Cr is 12 to 19% and an amount of Ni is 6 to 12% is described as follows. When the amounts of these elements are decreased to values lower than the above lower limits, it is impossible to provide a sufficient non-magnetic property, the specific magnetic permeability xcexc of which is not higher than 1.2. On the other hand, when the amounts of these elements are increased to values higher than the above upper limits, it is impossible to provide a sufficient magnetic flux density B4000 higher than 0.3T. Further, when an amount of Mn exceeds 2%, the working performance is deteriorated.
Mo and Nb are not necessarily added, however, Mo is effective to lower the point Ms, and Nb is effective to enhance the mechanical strength of the material. Therefore, according to an object, Mo or Nb may be added alone or together. In this case, when Mo exceeds 2% and Nb exceeds 1%, the working property is deteriorated. Therefore, it is preferable that the upper limit of Mo is 2% and the upper limit of Nb is 1%.
As described above, when not only the composition of each element is restricted but also the elements are appropriately combined with each other, it is possible to surely provide a high magnetic property.
When Hirayama""s Equivalent Heq is smaller than 20%, the specific magnetic permeability xcexc exceeds 1.2, and a sufficient non-magnetic property is not obtained. On the other hand, when Hirayama""s Equivalent Heq exceeds 23%, it is difficult for the magnetic flux density B4000 to exceeds 0.3T.
For the same reason as that of Hirayama""s Equivalent, the nickel equivalent Nieq is determined in a range from 9 to 12%, and the chromium equivalent Creq is determined in a range from 16 to 19%.
In this case, the material usually contains Si by an amount not more than 2% and Al by an amount not more than 0.5%, wherein Si and Al are contained as deoxidation elements, and also the material usually contains other impurity elements. However, there is no possibility that these elements deteriorate the property of the composite magnetic member.
Concerning the stainless steel produced in accordance with the first, second and third aspects described above, particularly the composite magnetic member, the shape may be formed into a cup shape, a cylindrical shape and a plate shape, etc., that is, it should be noted that the shape of the composite magnetic member is not particularly limited.
Fourth Aspect of the Invention
In order to accomplish the second object of the present invention, the present invention provides a method of producing a composite magnetic member comprising the steps of: forming an intermediately formed hollow body having a ferromagnetic portion and a non-magnetic portion, the non-magnetic portion contracting inward; and removing a residual tensile stress from the intermediately formed hollow body (claim 8).
The most remarkable point of this embodiment is that the embodiment includes a stress removing process in which a residual tensile stress is removed from the intermediately formed body. Conventionally, the intermediately formed body is used as a composite magnetic member as it is. However, according to the present invention, the stress removing process is added to the producing process of the composite magnetic member.
It is possible to use various stress removing processes, however, it is necessary that at least the residual tensile stress is relieved or removed. A compressive stress may be remained as a result of conducting the stress removing process. As a specific stress removing process, it is preferable to adopt a process in which a mechanical stress is given from the outside, the detail of which will be described later. Due to the foregoing, it is possible to remove a residual tensile stress without deteriorating the magnetic property of the above composite magnetic member.
Next, the mode of operation of this embodiment will be explained as follows.
According to the method of producing the composite magnetic member of the embodiment of the present invention, the aforementioned intermediately formed body is subjected to the above stress removing process. In this stress removing process, the residual tensile stress is sufficiently relieved or removed from the intermediately formed body. Therefore, the occurrence of stress corrosion cracks caused by a residual tensile stress can be surely prevented.
Consequently, according to this embodiment, it is possible to provide a method of producing a composite magnetic member having a high anti-stress corrosion property while the magnetic performance of the ferromagnetic portion and that of the non-magnetic portion are maintained.
Concerning the hollow shape of the intermediately formed body, it is sufficient that the intermediately formed body has a hollow portion inside. Examples of the shape of the intermediately formed body are a cylindrical shape having no bottom; and other shapes having bottom portions.
As shown in the embodiment according to claim 9, it is preferable that the cross-section of the intermediately formed hollow body is a U-shape. This shape is advantageous in that the intermediately formed hollow body can be easily subjected to clod drawing.
The following embodiment is a specific means for removing stress.
As described in the embodiment according to claim 10, it is preferable to produce a composite magnetic member as follows. In the stress removing process, a punch is forced or press-fitted into the above intermediately formed body so that the non-magnetic portion is expanded. After that, under the condition that the punch is inserted, the intermediately formed body is subjected to drawing with ironing so that the residual tensile stress can be changed into a residual compressive stress in the non-magnetic portion.
The most remarkable point of this embodiment is that the punch is forced into the intermediately formed body and then the intermediately formed body subjected to drawing with ironing as described above.
As described later, the intermediately formed body is provided in such a manner that after austenite alloy steel has been subjected to cold drawing so that it can be formed into a hollow shape, a portion of the hollow shape is subjected to high frequency induction heating. In other words, the non-magnetic portion can be formed as follows. Stress-induced martensite is generated by conducting cold working on the intermediately formed body, so that the intermediately formed body is made to be ferromagnetic. After that, a portion of the intermediately formed body is subjected to solution annealing, so that the portion can be returned from martensite to austenite. In this way, the non-magnetic portion can be formed.
In the intermediately formed body that has been made in the above manner, the non-magnetic portion is contracted inward as described above, and a residual tensile stress is generated in a portion close to the boundary between the non-magnetic portion and the ferromagnetic portion.
When the outer diameter of the intermediately formed body is determined, it is necessary to give consideration to an amount of reduction of the thickness caused in the process of ironing.
The punch used for expanding the non-magnetic portion and also used for conducting ironing is composed as follows. The outside diameter of the punch is the same as or slightly larger than the inside diameter of the main body of the intermediately formed body. Accordingly, when the punch is inserted into the intermediately formed body, it is closely contacted with the inner wall of the intermediately formed body.
When the above ironing is conducted, an ironing ratio is determined so that a residual tensile stress can be changed into a residual compressive stress in the intermediately formed body. However, when an ironing ratio is increased, that is, when a ratio of working is increased, the specific magnetic permeability xcexc of the non-magnetic portion increases, and its property is deteriorated. For the above reasons, it is necessary to give consideration so that the ratio of working is not increased too high.
Next, the mode of operation of this embodiment will be explained as follows.
According to the method of producing the composite magnetic member of this embodiment, after the intermediately formed body has been made, the punch is forced or press-fitted into it. Due to the foregoing, the non-magnetic portion is expanded and closely contacted with the outer circumference of the punch. At the same time, the ferromagnetic portion is also closely contacted with the outer circumference of the punch. Therefore, even if the inner diameters of the ferromagnetic portion and the non-magnetic portion fluctuate a little, the inner diameter of the thus obtained composite magnetic member can be made to be the same.
Next, while the punch is inserted into the intermediately formed body, it is subjected to ironing. Due to the above ironing, the thickness of the ferromagnetic portion can be made to be the same as the thickness of the non-magnetic portion. Therefore, the outer diameter of the ferromagnetic portion can be made to be the same as the outer diameter of the non-magnetic portion. When the above drawing with ironing is conducted, a ratio of drawing with ironing is determined so that a residual tensile stress can be changed into a residual compressive stress in the intermediately formed body and the property of the non-magnetic portion can not be deteriorated.
Therefore, a residual tensile stress can be changed into a residual compressive stress in the composite magnetic member while the magnetic properties of the non-magnetic portion and the ferromagnetic portion are maintained in the intermediately formed body.
For the above reasons, the stress corrosion-resistance property of the composite magnetic member can be sufficiently enhanced.
Next, as described in the embodiment according to claim 11, it is preferable that an ironing ratio is maintained at 2 to 9% in the process of ironing. Due to the foregoing, while the properties of the non-magnetic portion and the ferromagnetic portion are positively maintained in the intermediately formed body, a residual tensile stress can be changed into a residual compressive stress in the non-magnetic portion.
When the ratio of ironing is lower than 2%, there is a possibility that the residual tensile stress is not changed into the residual compressive stress. When the ratio of ironing exceeds 9%, there is a possibility that the specific magnetic permeability i of the non-magnetic portion increases and its property is deteriorated. In this connection, the ratio of ironing is expressed by (t0xe2x88x92t)/t0xc3x97100, wherein the thickness of material before conducting the ironing is to, and the thickness of material after the completion of working is t.
The following embodiment is another specific means for removing residual stress.
As described in the embodiment according to claim 12, in this process for removing residual stress, shot peening may be conducted on the inside or the outside of the above intermediately formed body where residual tensile stress has been generated. In this shot peening process, shot particles are made to collide with the inside or the outside of the above intermediately formed body.
In this case, the residual tensile stress can be greatly reduced or removed by the very simple process of shot peening. Therefore, it is possible to greatly enhance the anti-stress corrosion property while the production cost is maintained low.
According to the above method, shot particles are made to collide with a portion where tensile stress is given. Therefore, it is possible to reduce an intensity of residual tensile stress irrespective of the shape of the intermediately formed body.
When the above intermediately formed body having the ferromagnetic portion and the non-magnetic portion is produced, it is preferable that only a desired portion is heated so that the portion can be made to be non-magnetic after the material of the intermediately formed body has been subjected to cold drawing and made to be ferromagnetic. By the above method, it is possible to easily produce the above intermediately formed body, the magnetic property of which is high.
(Fifth Aspect of the Invention)
Another embodiment of the composite magnetic member produced by the above method is described as follows.
Another embodiment is a composite magnetic hollow member having a ferromagnetic portion and a non-magnetic portion.
Since this composite magnetic member is produced by the production process in which residual stress is removed, its stress corrosion cracking resistance property is very high as described above.
The cross-section of the hollow shape of the above composite magnetic member may be made to be a U-shape. In this case, it is preferable to compose this composite magnetic member in such a manner that the bottom side is formed into a ferromagnetic portion and the opening end side is formed into a non-magnetic portion. Due to the foregoing, the bottom side can be easily made to be ferromagnetic and the opening end side can be easily made to be non-magnetic.
(Sixth Aspect of the Invention)
The following is an embodiment of the invention which is an electromagnetic valve in which the above composite magnetic member, the magnetic property of which is high, is used.
The present invention provides an electromagnetic valve comprising: a coil for forming a magnetic circuit; a sleeve arranged in the magnetic circuit formed by the excitation of the coil; a plunger slidably arranged in the sleeve; and a stator arranged being opposed to the plunger via a moving space, wherein a fluid passage is opened and closed when the plunger is moved toward the stator by the excitation of the above coil, the sleeve is made of the composite magnetic member and a non-magnetic portion of the composite magnetic member is arranged so that the non-magnetic portion surrounds a moving space formed between the plunger and the stator.
The electromagnetic valve is one of the mechanical parts used for opening and closing a fluid passage of an automobile or other machines. Accordingly, there is a demand for high durability. In view of satisfying the demand for high durability, it is appropriate to use the composite magnetic member produced by the above method when the sleeve of the electromagnetic valve is made. That is, the thus made sleeve has a high anti-stress corrosion cracking property while it maintains a high magnetic property. For the above reasons, the durability of the entire electromagnetic valve into which this sleeve is incorporated can be greatly enhanced.
(Seventh Aspect of the Invention)
A method of producing a steel member comprising a non-magnetic portion and a magnetic portion, comprising the steps of a first step of cold rolling non-magnetic austenite steel to continuously form a ferromagnetic martensite elongated body; a second step of selectively annealing a predetermined portion of the elongated body corresponding to a non-magnetic portion to be formed; and a third step of forming said partially annealed elongated body into a shape and separating a steel member having a predetermined shape from said shaped elongated body.
When steel is subjected to cold rolling in the first step, stress inducing transformation of martensite occurs, so that the steel is made to be a structure of martensite. An elongated body is made of this ferromagnetic member. When annealing is partially conducted in the successive second step, a portion of the structure of martensite is returned to the structure of austenite, so that a non-magnetic portion is partially generated. In the third step, a member of steel, the shape of which is predetermined, can be completed by means of punching or cutting.
The remarkable point of this embodiment is described as follows. Predetermined portions of a ferromagnetic elongated body are successively annealed so that they can be changed into non-magnetic portions. After the formation of the non-magnetic portions, the members of steel, the shapes of which are predetermined, are successively separated.
In this embodiment, the elongated body is subjected to the first, the second and the third steps. Accordingly, the composite magnetic member can be easily mass-produced, and the productivity is high. Since annealing is conducted before forming (the third process), it is possible to form a non-magnetic member with high accuracy. Therefore, even small parts can be easily produced. Accordingly, even small members made of composite magnetic substance can be effectively mass-produced.
When the second step of annealing is conducted by irradiating laser beams, it is possible to form precise non-magnetic portions. In other words, when laser beams are utilized, it is possible to conduct a precise local annealing.
When the second step of annealing is conducted by high frequency induction heating, a thick plate can be subjected to a precise local annealing.
In the third step, it is preferable to adopt a separation method in which warm punching is conducted at a temperature in the range from 40xc2x0 C. to 600xc2x0 C.
When members are separated by means of punching, a minute amount of martensite (ferromagnetic portion) is generated in a small region of separation in which stress is acting. The thus generated ferromagnetic portion seldom affects the performance of a product, however, in the case of a small product, its performance is deteriorated. However, when warm punching is conducted at a temperature not lower than 40xc2x0 C., the generation of martensite can be suppressed, and it is possible to produce a highly accurate product. However, when the temperature exceeds 600xc2x0 C., the entire member becomes non-magnetic, and it is impossible to produce a member of steel composed of a non-magnetic portion and a ferromagnetic portion. For this reason, it is preferable to maintain the punching temperature in the range from 40xc2x0 C. to 600xc2x0 C.